1. Field of the Invention
The present invention relates to nanoelectronic devices, and in particular to nanostructured sensor systems for measurement of environmental gases, such as ammonia. Nanostructured sensor embodiments having aspects of the invention have utility in industrial, medical, and personal safety applications, including refrigeration leak detection, environmental management of poultry houses, and the like.
2. Description of Related Art
Ammonia has known toxic effects on both humans and animals even at low exposure levels. For example, occupational health regulations typically set upper limits for acceptable ammonia concentrations for human exposure. The limit in the UK is 25 ppm, in Sweden and Germany the limit is 25 and 20, respectively, for an 8-hour working day. Sweden also has a second limit of 50 ppm for a maximum of 5 minutes exposure.
In addition, ammonia is a common environmental contaminant in poultry houses with important consequences to poultry production. See, for example, I. Estevez, “Ammonia And Poultry Welfare”, Poultry Perspectives, Univ. of Maryland, Spring 2002 volume 4, issue 1, which publication is incorporated by reference.
Ammonia in poultry houses is difficult to avoid. Uric acid is excreted by poultry and, unless immediately removed, is decomposed by bacteria growing in the warm, moist conditions present in the poultry house litter. Under these conditions, uric acid is readily oxidized (enzymatic oxidative hydrolysis) to form urea as follows:C2(NH)4(CO)3+2H2O+1.5O2→2(NH2)2CO+3CO2  (1)Urea in turn is readily decomposed to form ammonia (hydrolysis by urease) as follows:(NH2)2CO+H2O→2NH3+CO2  (2)
The combination of ammonia and wet litter is responsible for a large number of health- and density-related welfare problems in poultry, for example, the occurrence of ascites, gastrointestinal irritation, and respiratory diseases is correlated with high levels of ammonia, as shown in TABLE 1.
TABLE 1Effect of ammonia levels on poultry healthLevel (ppm)Effects10Trachea irritation, susceptible to bacterial infections.20Increased rate of infection by Newcastle disease.25-75Impaired growth rate and feed conversion, reduced finalbody weight.25-50Air sac inflammation50Increased levels of keratoconjunctivitis.100Increased chick mortality.
In addition to the animal welfare consequences of the pain and stress of ammonia related pathologies, ammonia levels above about 25-50 ppm have an important affect on growth rate and feed conversion performance. See B. Lott, “Will ammonia really hurt broiler performance?”, Chicken Talk, Mississippi State University Extension, Information sheet No. 1639, 2003, which publication is incorporated by reference. It is believed that ammonia levels have a greater impact than behavioral factors in depressing poultry growth at high rearing densities (TABLE 2).
TABLE 2Effect of ammonia on average bodyweight of males at 7 weeks age.Ammonia (ppm)4 weeks (lb)7 weeks (lb)02.996.74252.956.55502.416.24752.476.23
Note that the effects of ammonia are highly dependent on exposure time. Therefore any effect demonstrated at rather high concentrations is likely to be present at much lower concentrations with longer exposure times. The cumulative effect of reduction in poultry growth and feed conversion due to environmental ammonia is a major economic burden on the poultry industry.
While ammonia levels may be partially controlled by attention to poultry diet, watering equipment, absorbent litter, and the like, adequate ventilation control is a necessary component of ammonia level management, as well as for temperature and humidity control. U.S. Pat. No. 5,407,129 issued to Carey et al. describes systems and methods for environmental control of poultry houses, and suggests that ammonia sensors may be included in such systems, which patent is incorporated by reference. However, in practice, systems have not been introduced into the poultry industry that use measurement of ammonia as a primary control variable, and instead typically seek to control ammonia indirectly by monitoring and controlling humidity and temperature.
A variety of different techniques for ammonia sensing are available for sensing ammonia, but generally suffer from one disadvantage or another. Colorimetric indicators or sensing paper do not provide an electronic signal suitable for feedback control systems. Metal-oxide and catalytic metal detectors have generally low selectivity, drift and a high operating temperatures (˜400-600 C). Optical gas sensors are generally large, expensive, and slow in response. Conducting polymer detectors have irreversible reactions, limiting their utility.
Current electrochemical sensors for ammonia have an electrolyte component that is consumed as the sensor is exposed to ammonia, thereby limiting service life, except in very low level exposure. The sensor life rating is thus in terms of “exposure time at concentration”. For example, for a sensor rating of 1000 ppm-day, an exposure to 50 ppm of ammonia (realistic level for poultry houses) for 20 days will completely exhaust the electrolyte.
What is needed is a low-cost, compact electronic sensor with dependable service life, stable calibration and high selectivity, to provide a practical ammonia sensor for medical, industrial and personal safety applications. In particular, there is a need for such sensors for environmental management of poultry houses and other livestock enclosures.